Evaluation of three-dimensional setup errors using cone beam computed tomography (CBCT) for mediastinal lymphoma treated with volumetric modulated arc radiotherapy (VMAT) and deep inspiratory breath-hold (DIBH) technique.

Background: Deep Inspiration Breath Holding (DIBH) for mediastinal lymphomas can potentially reduce late toxicities, especially cardiovascular and pulmonary toxicities, yet the geographical miss of target volume remains an area of concern. We conducted a study to derive optimal CTV-PTV margins for adequate coverage in mediastinal lymphomas treated with DIBH technique.

Methods and materials: Thirty patients with mediastinal lymphoma treated with radiotherapy using DIBH were included in this prospective study. Cone Beam Computed Tomography (CBCT) images were used as image guidance, and rigid registration was done with planning CT. The setup errors in anteroposterior (AP), supero-inferior (SI), and mediolateral (ML) directions were recorded. The random errors (σ) and systematic errors (Σ) were calculated using Stroom's and Van Herk's formulae to derive the PTV margins.

Results: Four hundred nine CBCT registrations were analyzed. The median number of registrations per patient was 14 (range: 5-25). The random errors were 0.227 cm, 0.308 cm, and 0.232 cm in AP, SI, and ML directions, respectively, while the systematic errors were 0.366 cm, 0.370 cm, and 0.316 cm, respectively. The CTV-PTV margins were 0.7 cm (AP), 0.9 cm (SI), and 0.7 cm (ML) as per Stroom's and 0.8 cm (AP), 1.0 cm (SI), 0.8 cm (ML) as per Van Herk's formulae.

Conclusion: An asymmetric PTV margin of 1.0 cm longitudinally and 0.8 cm both anterior-posteriorly and laterally is adequate to cover setup uncertainties for patients treated with mediastinal radiotherapy using the DIBH technique at our center. The setup margins can further be reduced with daily image guidance and an online correction protocol.